Even though current technologies of fiber laser have made significant progress toward achieving a compact and reliable fiber laser system providing high quality output laser with ever increasing output energy, however those of ordinary skill in the art are still confronted with technical limitations and difficulties. Specifically, in a fiber laser system implemented with the Chirped Pulse Amplification (CPA) for short pulse high power laser amplifier, the CPA systems are still limited by the technical difficulties that a short pulse laser with an ultra-high energy over 10 mJ up to 100 mJ cannot be easily generated. There are several technical problems that limit the generation of such ultra-high energy lasers. A first technical confronted those of ordinary skill in the art is the related to the high repetition rate of a mode-locked (ML) oscillator, conventionally 40˜100 MHz. Under certain average power, it is hard to get very high pulse energy if one keeps such a high repetition rate. In a typical short-pulse high-energy fiber laser system, the idea of Chirped Pulse Amplification (CPA) is widely implemented. Basically it consists of four parts: a mode-locking (ML) oscillator for providing short laser pulse, a stretcher to get long pulse duration, an amplifier to amplify the laser pulses to a high energy, and a compressor to get short pulse and high peak power. For a lot of applications, high pulse energy and peak power is more interested instead of high repetition rate and/or high average power. In fiber laser system, if the laser system provides an option for selecting some pulses from the high repetition rate ML oscillator as the target pulse for amplification under same average power and/or same pumping level, the laser system is able to amply these picked-up pulses with much higher energy and peak power. Another technical difficulty of a fiber based laser system with short pulses is the limitations due to the nonlinear effects, such as third order dispersion (TOD) cased in fiber based pulse streching, self-phase modulation (SPM), and stimulated Raman effects (SRS).
In order to resolve the high repetition rate (tens of MHz) limitation, a pulse picker as that shown in FIG. 1 is implemented to achieve high peak power in a fiber laser system. In order to suppress the ASE noise and reduce the distortions after the pulse is amplified, a pulse is amplified before the amplified pulses are projected into a pulse picker. This configuration leads to a more complicated system and a higher cost. As an example, the average power from the fiber ML seed is a few mille watts, without the preamplifier, the average power after the pulse picker would be around 1 micro watt, or even lower, which can not dominate the ASE noise in the high gain amplification chain. Even with the preamplifier with an output of <400 mW, which is mainly limited by single mode pump diode, the average power after the pulse picker is still below 0.5 mW, which requires a delicate and elaborate design for the amplification chain.
On the other hand, the free space short pulse solid-state laser system, mJ to Joule level energy has been generated, for 10 Hz to 1 KHz repetition rate, this corresponds to 1˜10 W average power, which does not introduce very serious thermal issue even for room temperature laser amplifier. Higher average power requires cryogenically cooled system. Meanwhile, the fiber laser system can easily get over 100 W average power without a requirement to deal with the thermo-optic effects. However, for the fiber system, the tight confinement of laser light in a small core makes it very hard to achieve high peak power due to nonlinear effects.
Therefore, a need still exists in the art of designing and configuring a new and improved laser system with a new configuration and method to provide ultra-high energy short-pulse lasers such that the above-discussed difficulty may be resolved.